The invention relates to steering and thrust control systems for waterjet driven boats.
With a waterjet drive, seawater is drawn in through the bottom of the boat and ejected in a stream out the back. The reaction to this movement of water is the propulsive force that moves the boat. Near the back of the stream is a nozzle, which serves two functions. It accelerates the stream by reducing its diameter, and it can be turned from side to side to deflect the exiting stream to apply a component of side force on the aft part of the boat. The nozzle is to a jet what a rudder is to a boat equipped with conventional propellers. Both are typically connected to a steering wheel.
The aftmost portion of the jet, just behind the nozzle, is a device called a reversing bucket. Its function is to allow the operator to reverse some or all of the stream in order to stop or back up the boat. In normal underway operation the bucket is elevated above the stream and has no effect. When reduced forward thrust is desired the bucket can be lowered into the stream, forcing a portion of the flow through curved channels until it exits in a forward and slightly downward direction. When roughly half the stream is still streaming aft below the bucket and half is being reversed to a more forward direction (the neutral bucket position), an approximate balance point can be reached that results in approximately no forward or aft thrust on the boat. If the bucket is lowered to the full down position, nearly all the thrust is reversed and the boat should begin moving in reverse. The particular design of some reverse buckets (e.g., Hamilton waterjets), and the way the bucket interacts with the nozzle, permits a net thrust in any direction in the plane of the water's surface. Side to side force is adjusted by nozzle position, and forward or aft force by bucket position.
A waterjet is either engaged and pumping water or disengaged and not pumping water. It does not ordinarily have a forward and reverse in the same manner as a conventional propeller. A transmission with reverse gear can be provided as a means of allowing the engine to run without engaging the jet and to allow for backflushing that results from reversing the drive shaft to the jet to clear an obstruction that may have been drawn against the jet inlet. Actual reverse thrust is accomplished with the jet engaged in the forward direction and the bucket lowered, similar in concept to the reversing arrangement on aviation jet engines.
Waterjet drives have numerous advantages, e.g., low draft, reduced noise, improved high-speed maneuverability. But they can make a boat difficult to control at slow speeds in tight quarters (e.g., when docking). The reason for this is that, heretofore, there has been no simple way to achieve zero thrust or zero side force. In a conventionally powered boat, zero thrust and zero side force are easily achieved, simply by putting the transmission into neutral, thereby bringing the propeller to rest. But with a waterjet, the only way to achieve zero thrust is to move the bucket to a position at which the net of the forward and reverse portions of the jet is balanced. That position can only be chosen approximately. It takes considerable training and experience for an operator to acquire a sense of what the waterjet drive is doing, to allow successful slow speed operation.
Waterjet drives also behave differently in reverse from propeller driven craft. Because the flow of water through the jet is always in one direction, deflection of the stream results in the same sideward force regardless of whether the boat is moving forward or in reverse. This is in contrast to a conventional rudder, whose effect on the stern of a boat is reversed depending on the direction of travel through the water. This difference in steering in reverse presents difficulties for new operators, who anticipate that steering direction will change when the boat is backing up.
To control movement of the bow of a boat, some boats are equipped with bowthrusters. Such a thruster is often installed in a tube that runs from side to side at the bow below the waterline. In the middle of this tube is a propeller that can thrust either way by reversing rotation. In smaller boats, this propeller is usually driven by an electric motor. The combination of waterjet and bowthruster can give a boat extraordinary maneuverability. Movement in any direction in the plane of the water's surface is possible, even directly sideways. But, unfortunately, the operator is typically required to skillfully coordinate different controls simultaneously to take full advantage of this maneuverability. E.g., a foot pedal or left/right deflection of a hand-operated lever may be used to control the bowthruster, a steering wheel, to control the rear nozzle, and a throttle lever, to control speed.
Some very large waterjet driven ships have solved the zero thrust difficulty by controlling the waterjet with an inertial control system that senses applied thrust (e.g., using accelerometers), and adjusts the waterjet bucket position until a desired thrust level is achieved. When the operator desires a zero thrust level, the control system adjusts the bucket position until the inertial sensors detect zero applied thrust. This solution is too expensive for small boats (i.e., boats 75 feet or less in length).